1. Field of the Invention
This invention relates to an improved system of a thermal barrier coating (TBC) having a metallic bond coat and a thick dual-constituent top coat. In particular according to the present invention the two constituents of the top coat are separated by a graded interface which leads to an increase in the thickness of the top coat and improved quality of the overall TBC system.
2. Description of the Prior Art
It is accepted practice in the gas turbine engine industry to apply a TBC (typically an MCrAlY metallic bond coat layer followed by a ceramic partially-stabilized zirconia top coat layer) onto hot section components, to prolong their lives. Examples of components currently coated with TBC include combustor liners, transition ducts and first stage blades and vanes. U.S. Pat. No. 5,384,200 issued Jan. 24, 1995 discloses an example of such TBC where both the metallic and the ceramic layers of the TBC may be deposited by atmospheric plasma spray.
Applicant's own Canadian Patent Application No. 2,211,961 filed Jul. 29, 1997, discloses the possibility of using vacuum plasma spray (VPS) in the formation of the TBC on a structural superalloy layer of a combustion system component, and also the possibility of having a dual-constituent top coat in such TBC.
Moreover, it is also known to produce a coating with a continuous compositional gradient by co-depositing at least two powders onto a substrate by feeding them at separately controllable variable feed rates into a plasma torch. This is disclosed, for example, in U.S. Pat. No. 5,362,523 of Nov. 8, 1994. However, such graded coatings are not used as part of a TBC having a metallic bond coat and a ceramic top coat that are normally used to protect gas turbine engine components.
Current TBC systems widely used to protect gas turbine engine components include a VPS applied MCrAlY bond coat (typically.about.75-125 .mu.m thick) followed by an atmospheric plasma sprayed (APS) yttria partially-stabilized zirconia top coat (typically.about.125-375 .mu.m thick). This provides a temperature drop across the TBC of approximately 100 to 150.degree. C. In addition to the TBC, components in the hot section normally require some cooling to further mitigate overheating. Much of the improvements to the turbine performance efficiency is directly related to the ability of increasing the allowable combustor and turbine entry temperature (TET).